1. Field of Invention
This invention relates generally to wind driven power generation plants and, more particularly, but not by way of limitation, to power plants which utilize windmills having automatic rotation rate controls.
2. Brief Description of the Prior Art
The use of windmills as a source of energy has long been known, examples of which are well-known windmills of the Netherlands and, closer to home, the small windmills that dot the rural American landscape. While such windmills have proven to be effective means of accomplishing the tasks for which they are designed, they do not have the characteristics necessary for their inclusion in a power distribution grid. In particular, the rotation rate of such windmills is usually uncontrolled because their use is such that rotation rate control is unnecessary. A windmill of this type is generally used to drive a mechanical system which carries out a desired function at the site of the windmill and the rate at which the mechanical system is driven is usually not critical.
On the other hand, rotation rate control is an essential element of a windmill designed for the generation of electric power. As is well-known, generators are based on Faraday's law of induction which states that the electromotive force induced in a conducting loop is proportional to the time rate of change of the flux of the magnetic induction passing through the loop. In a generator, regardless of the specific details of its design, one or more windings are arranged so that the rotation of the shaft of the generator will result in a corresponding cyclic variation of the flux of the magnetic induction in the windings. Since, in accordance with Faraday's law, this cyclic rate is reflected both in the frequency and the amplitude of the output of the generator, the output of the generator will depend on the rotation rate of the generator shaft through the correspondence between the rotation rate of the shaft and the frequency of the cyclic rate at which the flux of the magnetic induction in the generator windings is varied.
In order to permit the efficient generation and widespread distribution of electricity, the frequency and amplitude requirements of electric motors used in industrial machinery and in home appliances have been standardized and electric power generation power plants and distribution grids are designed to supply the requirements of such motors. A wind driven power generation system must meet the same requirements if it is to contribute to the power generation capacity of a power distribution grid. Accordingly, rotation rate control is a necessity.
The rotation rate control requirement has been recognized in various United States Patents concerning wind driven electric power generators. In particular, U.S. Pat. No. 1,523,295, granted Jan. 13, 1925, to James T. Ryan, discloses a controlled speed windmill which uses a vertically disposed rotor having blades which are partially shielded in order to provide an imbalance in wind effect on the blades to turn the rotor. Ryan teaches that rotation rate control may be achieved by varying the position of the shield in response to the wind speed so that the imbalance of the wind effect on the rotor is maintained substantially constant despite variations in the speed of the wind. In particular, Ryan discloses a shield having a vane pivotally mounted thereon. The position of the shield is determined by the relative orientations of the vane and the shield and this orientation is, in turn, determined by the rotation rate of the rotor via the use of a mechanical governor mounted on the rotor and interacting with the vane.
A second requirement for windmills to be used as part of the generating system which provides electricity for a power distribution grid is large size. While the wind provides a source for a large amount of energy, this energy is distributed over a correspondingly large area of the earth's surface. For example, the net wind energy crossing an area of one square meter disposed perpendicularly to the direction of the wind is only about two kilowatts when the wind speed is 15 meters per second; that is, when the wind speed is slightly more than 30 miles per hour. Since modern power plant capacities are in the tens or hundreds of megawatts range, it is clear that even if a wind driven power generation system could capture the total energy content of the wind to which it is exposed, it would have to expose a large area to the wind to be competitive with other types of power generation systems.
The necessary size of economically feasible wind driven power plants for the generation of electric power introduces technical problems which call for qualitatively new solutions. In particular, it is not possible to build plants which are competitive with other power generating systems by merely scaling up the small wind generation systems which are known in the art. Construction means suitable for a small wind generator would require massive support structures if applied to a very large windmill so that the capital investment in such a windmill would be prohibitive.
Moreover, control systems suitable for small windmills may not be suitable for a large windmill. In particular, a mechanical governor such as that used by Ryan, U.S. Pat. No. 1,523,295, while effective for the control of moderately sized windmill, is not suitable for a windmill of the size contemplated in the present invention. The reason for this is not difficult to discern. A mechanical governor mounted directly on the rotor of a windmill and providing direct adjustment of the control means of the windmill depends for its effectiveness on a relatively high rotation rate of the rotor. Large windmills are characterized by low rotor rotation rates. For example, if the tangential speed of a point on the rotor located a distance of 15 meters from the axis of rotation thereof is 15 meters per second, that is, approximately 30 miles per hour, the rotation rate of the rotor is less than 10 revolutions per minute. A reasonably sized mechanical governor operating at such a low rotation rate is incapable of providing a force of large enough magnitude to directly actuate the large control mechanisms found in large wind driven electric power plants.